PCF8563's second/minute/hour/day/weekday/month/year registers are
contiguous (0x02-0x08), so PCF8563_ReadDateTime() bursts all 7 bytes
in one transaction. PCF8563_ReadTime() becomes a thin wrapper that
discards the date fields. WriteTime() is kept as its own 3-byte-only
implementation (writing only sec/min/hour) rather than routed through
the new WriteDateTime(), since forcing it through a 7-byte write would
require synthesizing date values and risk corrupting the stored date;
WriteDateTime() is added as a separate full 7-byte write for callers
that actually have real date+weekday values to persist.
